Scattering of vector mesons off nucleons

We construct a relativistic and unitary approach to 'high' energy pion- and photon-nucleon reactions taking the $πN, πΔ$, $ρN$, $ωN$, $ηN, K Λ, K Σ$ final states into account. Our scheme dynamically generates the s- and d-wave nucleon resonances N(1535), N(1650) and N(1520) and isobar resonances $Δ(1620)$ and $Δ(1700)$ in terms of quasi-local interaction vertices. The description of photon-induced processes is based on a generalized vector-meson dominance assumption which directly relates the electromagnetic quasi-local 4-point interaction vertices to the corresponding vertices involving the $ρ$ and $ω$ fields. We obtain a satisfactory description of the elastic and inelastic pion- and photon-nucleon scattering data in the channels considered. The resulting s-wave $ρ$- and $ω$-nucleon scattering amplitudes are presented. Using these amplitudes we compute the leading density modification of the $ρ$ and $ω$ energy distributions in nuclear matter. We find a repulsive energy shift for the $ω$ meson at small nuclear density but predict considerable strength in resonance-hole like $ω$-meson modes. Compared to previous calculations our result for the $ρ$-meson spectral function shows a significantly smaller in-medium effect. This reflects a fairly small coupling strength of the N(1520) resonance to the $ρN$ channel.

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